Underneath the LCD screen are a few test points. Two of them are helpfully labeled TXD (transmit data) and RXD (receive data). The former is for data coming out and the latter is data going in. Solder some wires to these points. Pro tip: Use different coloured wires so you can remember which is TX and which is RX.
Extra tip: Some Dingoos have this empty U5 position. In the top-right corner of the pic you can see I’ve soldered a smd led in series with a 47ohm resistor, which will give me a power-light. 🙂 Pretty convenient.